Erectile dysfunction (ED) is defined as the inability to achieve or maintain an erection sufficiently rigid for satisfactory sexual intercourse. Currently it is estimated that approximately 7-8% of the male population suffer from some degree of ED, the equivalent of at least 20 million men in the United States alone. Since the likelihood of ED increases with age, it is projected that the incidence of this condition will rise in the future as the average age of the population increases.
Male erectile dysfunction may be the consequence of psychogenic and/or organic factors. Although ED is multi-factorial, certain sub-groups within the male population are more likely to present with the symptoms of the disorder. In particular, patients with diabetes, hypertension, heart disease, and multiple sclerosis have a particularly high prevalence of ED. In addition, patients who take certain classes of drugs such as antihypertensives, antidepressants, sedatives, and anxiolytics are more prone to suffer from ED.
Treatments for ED include a variety of pharmacologic agents, vacuum devices, and penile prostheses. Among the pharmacologic agents, papaverine, phentolamine, and alprostadil are currently used in practice. These agents are only effective after direct intracavernosal or intraurethral injection, and are associated with side effects such as priapism, fibrosis, penile pain and hematoma at the injection site. Vacuum devices are a noninasive alternative treatment for ED. These devices produce an erection by creating a negative pressure around the shaft of the penis resulting in an increased blood flow into the corpus cavernosum via passive arterial dilation. Although this form of therapy is frequently successful in ED of organic origin, complaints include the lack of spontaneity and the time involved in using a mechanical device, and difficulty and discomfort with ejaculation. A variety of semi-rigid or inflatable penile prostheses have been used with some success, particularly in diabetic men. These devices are generally considered when other treatment options have failed, and are associated with an increased risk of infection and ischemia.
Recently, the phosphodiesterase V (PDEV) inhibitor, sildenafil (Viagra.RTM.) was approved by the FDA as an orally effective medication for the treatment of ED. Sildenafil, 5-[2-ethoxy-5-(4-methylpiperazin-1-ylsulphonyl)phenyl]-1-methyl-3-n-propyl -6,7-dihydro-1H-pyrazolo[4,3-d]pyrimidin-7-one and a number of related analogs and their use as antianginal agents are described in U.S. Pat. Nos. 5,250,534 and 5,346,901. The use of sildenafil and related analogs for treating male erectile dysfuntion is described in PCT International Application Publication No. WO 94/28902, published Dec. 22, 1994. In clinical studies, the drug improved sexual function in about 70% of the men who suffer from ED of psychogenic or organic etiology. However, the drug showed less dramatic efficacy in patients who had undergone a radical prostatectomy, with improved erections in 43% of patients who took sildenafil versus 15% on placebo. In addition, the use of sildenafil is associated with several undesirable side effects including headache, flushing and disrupted color vision which result from non-selective effects on a variety of tissues. In spite of these shortcomings, the drug is viewed by patients as preferable to other treatments which involve the introduction of medication directly into the penis via injection, the use of an external device or a surgical procedure.
Dinh et al. (J. Org. Chem. 1975, 40, 2825) describe the preparation of 5-hydroxymethyl-2-furanyl and 5-benzyloxy-2-furanyl pyrazolo[4,3-d]pyrimidin-7-ones as by-products and intermediates in the preparation of analogs of formycin B.
Bacon et al., in U.S. Pat. No. 5,294,612, describe a series of 6-heterocyclyl pyrazolo[3,4-d]pyrimidin-4-one phosphodiesterase inhibitors and their use in treating cardiovascular disease.
Sexually stimulated penile erection results from a complex interplay of physiological processes involving the central nervous system, the peripheral nervous system, and the smooth muscle. Specifically, release of nitric oxide from the non-adrenergic, non-cholinergic nerves and endothelium activates guanylyl cyclase and increases intracellular cGMP levels within the corpus cavernosum. The increase in intracellular cGMP reduces intracellular calcium levels, resulting in trabecular smooth muscle relaxation, which, in turn, results in corporal volume expansion and compression of the sub-tunical venules leading to penile erection.
Agents that increase the concentration of cGMP in penile tissue, either through enhanced release or reduced breakdown of cGMP, are expected to be effective treatments for ED. The intracellular levels of cGMP are regulated by the enzymes involved in its formation and degradation, namely the guanylate cyclases and the cyclic nucleotide phosphodiesterases (PDEs). To date, at least nine families of mammalian PDEs have been described, five of which are capable of hydrolyzing the active, cGMP, to the inactive, GMP, under physiological conditions (PDEs I, II, V, VI, and IX). PDE V is the predominant isoform in human corpus cavernosum. Inhibitors of PDEV, therefore, would be expected to increase the concentration of cGMP in the corpus cavernosum and enhance the duration and frequency of penile erection.
PDEV also has been found in human platelets and vascular smooth muscle, suggesting a role for this enzyme in the regulation of intracellular concentrations of cGMP in cardiovascular tissue. In fact, inhibitors of PDEV have been shown to produce endothelial-dependent vasorelaxation by potentiating the increases in intracellular cGMP induced by nitric oxide. Moreover, PDEV inhibitors selectively lower the pulmonary arterial pressure in animal models of congestive heart failure and pulmonary hypertension. Hence in addition to their utility in ED, PDEV inhibitors would likely be of therapeutic benefit in conditions like heart failure, pulmonary hypertension, and angina.
Accordingly, it is an object of the invention to identify compounds which inhibit phosphodiesterases, specifically PDEV. It is another object of the invention to identify compounds which are useful for the treatment of sexual dysfunction, particularly erectile dysfunction in male animals. Still another object of the invention is to identify methods for treating sexual dysfunction, especially erectile dysfunction, using the compounds of the present invention.
We now describe a series of 5-heterocyclyl-pyrazolyl[4,3-d]pyrimidin-7-ones with the ability to inhibit phosphodiesterase type 5 in an enzyme assay.